Large commercial and military vessels are faced with meeting increasingly-strict engine exhaust emissions standards. Applicable regulatory standards include the Marine Pollution (MARPOL) Annex VI and United States Environmental Protection Agency (USEPA) Tier 1-3, MARPOL, also known as the Ship Pollution Protocol, regulates various aspects of solid, liquid, and atmospheric pollution by ships. 33 U.S.C. §§1901 et seq, (2000). MARPOL Annex VI addresses atmospheric pollution. Applicable EPA regulations generally limit exhaust emissions of NOx, PM, carbon monoxide, and unburned hydrocarbons (HC). See 40 C.F.R. § 94 (1999). The use of a wet emissions reduction system reduces the amount of NOx, and in some cases, the amount of PM created by an internal combustion engine.
Large commercial and military vessels use large diesel engines for power. Diesel engines are governed, not throttled like gasoline engines. A diesel engine does not have a throttle plate like a gasoline engine, which restricts the intake of air, so the cylinders are completely filled with fresh air on each intake stroke. This means that rather than controlling the amount of both air and fuel available to the engine, the engine's output is regulated exclusively by the amount of fuel that is injected into the engine cylinder. Therefore, diesel engines run at a wide variety of fuel/air ratios from very lean (approximately 60-100:1) at idle to only slightly lean at full power (approximately 18:1). For a number of reasons, when running rich or even close to stoichiometric, diesel engines tend to produce an undesirable amount of smoke in the exhaust due to unburned fuel.
Running the engine lean to optimize efficiency and minimize smoke however, produces a different, and undesirable, effect. At standard conditions, nitrogen is a fairly inert diatomic gas. However, at the very high temperatures found in a diesel engine combustion chamber, the excess thermal energy breaks apart both diatomic nitrogen and diatomic oxygen. As a result, because there is not enough fuel to consume all of the oxygen in a given charge due to the lean air/fuel ratio, the remaining oxygen combines with the (now unstable) nitrogen atoms to form NOx, one of the targets of emissions regulations.
However, the use of a wet emissions reduction system in an internal combustion engine lowers the temperature in the combustion chamber. The conversion of the injected water into steam absorbs the residual heat in the combustion chamber from the previous combustion event and lowers the ultimate temperature of the current combustion event. This reduction in temperature reduces the amount of thermal energy available in the combustion chamber and thus reduces the formation of NOx. The injected water molecules may also provide a greater surface area for fuel molecules to cling to, allowing for more homogenous dispersion and combustion throughout, the combustion chamber. This results in a more complete combustion event and may decrease the amount of particulate matter exhausted into the atmosphere.
The use of a wet emission reduction system also reduces the possibility of pre-detonation. Pre-detonation occurs when the residual heat in the combustion chamber, coupled with rising pressure in the combustion chamber as the piston approaches top dead center (TDC), causes the fuel to ignite explosively and prematurely (ideal combustion event timing being somewhere just before or just after TDC depending on conditions). While the problem is most associated with gasoline engines, pre-detonation does occur in diesel engines as well. Pre-detonation is generally the source of the distinctive “knocking” sound associated with large diesel engines at idle (when diesels are running at their leanest settings).
Pre-detonation also results in the formation of NOx. Pre-detonation causes an explosive ignition of the fuel/air mixture rather than a controlled burn. This creates very high local combustion pressures and temperatures, which promote the formation of NOx for the same reasons outlined above. Pre-detonation also causes incomplete combustion in other local areas as the explosion “blows out” the flame front. This prevents the flame front from propagating through the combustion chamber in the manner necessary for complete combustion. The resulting incomplete combustion leads to HC, another subject of emissions regulations, which are then exhausted into the atmosphere. Pre-detonation also results in reduced efficiency of the diesel engine while under load because of the effects described above.
The “x” in NOx indicates that at these high temperatures and energy levels there are several unstable compounds that can be formed with varying numbers of oxygen atoms. These molecules are unstable, however, and can only form due to the abundance of energy available. This instability results in compounds that are very reactive and thus damaging to the environment, people, and animals. Many oxides of nitrogen are colorless and odorless; however, nitrogen dioxide is a major component in the reddish-brown layer of air over many urban centers. NOx is also a major contributor to ground level ozone, acid rain, and global warming.
The wet emissions reductions systems referred to above can be any of several types currently in use and under development. The systems include, but are not limited to, an intake manifold water injection system, a water/fuel emulsion system, or a direct water injection system. Running internal combustion engines with any of these wet emissions reduction systems has several benefits in addition to reduced emissions. First, the increased humidity in the combustion chamber may have a lubricating effect, thus lowering wear on the cylinder walls and piston rings. Second, lowering combustion temperatures reduces the overall thermal stress on the engine as well as minimizing localized “hot spots.” Third, the conversion of water to steam in the combustion chamber tends to remove carbon deposits and prevent their further formation both in the combustion chamber and throughout the exhaust system of the diesel engine. Reduction in carbon deposits can, in turn, reduce required maintenance, extend lubricating oil life, and make turbochargers and economizers more efficient.
Wet emissions reduction systems, however, require large amounts of water to obtain optimum results. Indeed, the amount of water needed for optimum effect may exceed 20% of the fuel requirement. One available source of abundant water aboard large passenger vessels, especially modern cruise ships, ferries, and, potentially, large naval vessels, in adequate quantities for consideration for use with these emission reduction technologies is AWTS clean effluent. In these systems, vessel black water and/or grey water is treated and purified to a high grade of purity that typically meets or exceeds the standards found in 33 C.F.R. §159 (e) (1999). These systems typically employ tertiary (or higher) waste water treatment technologies.
AWTS clean effluent commonly needs removal of some chemical and solid content, specifically nutrients, phosphorous, ammonia, and suspended solids, to meet purify requirements for engine applications. On vessels with property functioning AWTS, AWTS clean effluent is generally plentiful. In these cases, the water is recycled by the present invention and therefore has little associated direct cost, because it would otherwise have been disposed of after treatment by the AWTS. Additionally, recycling the water potentially relieves vessel effluent disposal problems and reduces treated wastewater discharges to the environment. As a result, the AWTS is used to reduce both water pollution discharges and air pollution discharges.
The AWTS effluent is further purified and filtered by the present invention for use in a wet emissions reduction system. The novel use of AWTS effluent in the methods and systems of the present invention assists ship owners and operators in meeting USEPA and MARPOL emissions requirements. Additionally, the systems and methods of the present invention reduce the maintenance and increase the longevity of the vessel's engines as well as providing an alternate means of disposal for the AWTS clean effluent.